1. Field of the Invention
The present invention is concerned with adhesives made from vegetable or grain proteins. More particularly, the present invention is concerned with such adhesives wherein the adhesives have been combined with an ingredient and/or pH level that modifies the adhesive's properties. Still more particularly, the present invention is concerned with adhesives that have been modified with an ingredient containing at least one carboxyl group, and/or at least one aldehyde group, and/or at least one epoxy group. Still more particularly, the modified adhesive will be used near the isoelectric point of the protein used therein.
2. Description of the Prior Art
It is estimated that demand for adhesives in the US will reach 15.2 billion pounds in 2004. Up to 60% of these adhesives are used in the wood industry for plywood, particle board, and furniture applications. Most of these adhesives, including hot melt, emulsion, vinyl-based, rubber-based, acrylics, phenolic, amino, epoxy, and silicone adhesives, are from petroleum resources. Urea formaldehyde (UF), having a typical wet strength of 3.5 MPa, 70% wood failure, and 0 MPa boiling strength, and phenol formaldehyde (PF), having a typical wet strength of 3.5 Mpa with 81% wood failure, and 2.7 MPa boiling strength with 72% wood failure, are commonly used conventional adhesives. However, concerns have been raised in recent years about naturally limited petroleum resources, environmental pollution, and health problems caused by manufacturing, transportation, storage, and utilization of those petroleum-based adhesives. Accordingly, development of environmentally friendly adhesives from renewable resources is becoming increasingly important.
Soy protein has great potential to be an alternative to the petroleum-based adhesives because of its abundance, renewability, and biodegradability. Soy based adhesives were first developed around 1923 when a patent was granted for a soy meal-based glue. However, soy protein adhesives have suffered from low gluing strength and low water resistance. In recent years, some efforts have been made to improve their adhesion strength and water resistance. Hettiarachchy et al. used an alkali-modification method and obtained adhesives with improvement in both characteristics. Sun and Bian found that urea-modified soy protein adhesives were more water-resistant than those modified by alkali. Other modification agents included trypsin, guanidine hydrochloride (GuHCl), sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate, and 1,3-dichloro-2-propanol (DCP) [2,6-8]. The DCP was believed to form epichlorohydrin under alkali conditions that could further react with amino, hydroxyl, and carboxyl groups. GuHCl and SDS were also used to modify soy protein adhesive for fiberboard application. Li et al used a cationic polymer, polyamide-epichlorohydrin (PAE), to cross-link soy protein and got improved adhesion strength and water resistance. The improved properties were attributed to chemical reactions between PAE and soy protein at elevated temperature. PAE is a well-known wet-strength resin, which is widely used in paper and pulp industry. It is prepared by reaction of water-soluble polyamide with epichlorohydrin, to form water-soluble, cationic polymer with reactive azetidinium groups. The azetidinium group can react with active-hydrogen groups, such as carboxyl, hydroxyl, and amino functional groups. These reactions can increase water resistance by forming an insoluble network coating around surface and by forming cross-linkages between resin and substrate.
Accordingly, what is needed in the art is an adhesive derived from a renewable source, which has adhesion and water resistance properties comparable to conventional petroleum-based adhesives, and methods of making the same. What is further needed is a protein-based adhesive that has similar water resistance and adhesion properties as conventional petroleum-based adhesives, and methods of making the same. What is further needed are adhesives derived from vegetable or grain proteins that have been modified to increase their adhesive strength and water resistance, and methods of making the same. Finally, what is needed are vegetable protein based adhesives modified with compounds containing carboxyl groups, and/or aldehyde groups, and/or epoxy groups, wherein the adhesive is prepared at or near the isoelectric point of the vegetable protein.